How did The FBI build its informant network and is it dependable?

So how did the FBI build its informant network? It began by asking where US Muslims lived. Four years after 9/11, the bureau brought in a CIA expert on intelligence-gathering methods named Phil Mudd. His tool of choice was a data-mining system using commercially available information, as well as government data such as immigration records, to pinpoint the demographics of specific ethnic and religious communities—say, Iranians in Beverly Hills or Pakistanis in the DC suburbs.

The FBI officially denies that the program, known as Domain Management, works this way—its purpose, the bureau says, is simply to help allocate resources according to threats. But FBI agents told me that with counterterrorism as the bureau's top priority, agents often look for those threats in Muslim communities—and Domain Management allows them to quickly understand those communities' makeup. One high-ranking former FBI official jokingly referred to it as "Battlefield Management."Some FBI veterans criticized the program as unproductive and intrusive—one told Mudd during a high-level meeting that he'd pushed the bureau to "the dark side." That tension has its roots in the stark difference between the FBI and the CIA: While the latter is free to operate internationally without regard to constitutional rights, the FBI must respect those rights in domestic investigations, and Mudd's critics saw the idea of targeting Americans based on their ethnicity and religion as a step too far. People cooperate with law enforcement for fairly simple reasons: ego, patriotism, money, or coercion. The FBI's recruitment has relied heavily on the latter. One tried-and-true method is to flip someone facing criminal charges. But since 9/11 the FBI has also relied heavily on Immigration and Customs Enforcement, with which it has worked closely as part of increased interagency coordination. A typical scenario will play out like this: An FBI agent trying to get someone to cooperate will look for evidence that the person has immigration troubles. If they do, he can ask ICE to begin or expedite deportation proceedings. If the immigrant then chooses to cooperate, the FBI will tell the court that he is a valuable asset, averting deportation. 
Once someone has signed on as an informant, the first assignment is often a fishing expedition. Informants have said in court testimony that FBI handlers have tasked them with infiltrating mosques without a specific target or "predicate"—the term of art for the reason why someone is investigated. They were, they say, directed to surveil law-abiding Americans with no indication of criminal intent."The FBI is now telling agents they can go into houses of worship without probable cause," says Farhana Khera, executive director of the San Francisco-based civil rights group Muslim Advocates. "That raises serious constitutional issues."http://motherjones.com/politics/2011/08/fbi-terrorist-informants